A number of techniques have been used for liquid purification including: filtration, chemical sterilization, oxygenation, chemical flocculation, ultraviolet sterilization, and reverse osmosis.
Prior processes have used combinations of the above mentioned purification techniques. For example, U.S. Pat. No. 3,505,215 by Bray, issued Apr. 7, 1970, discloses the use of a vessel containing a spiral wound reverse osmosis membrane preceded and followed by vessels containing filters. U.S. Pat. No. 4,169,789 by Lerat, issued Oct. 2, 1979, discloses a sea water purification process whereby the water is pumped through a series of four or more separate filters at a relatively low pressure and then into a separate reverse osmosis unit at a relatively high pressure. U.S. Pat. No. 4,366,063 by O'Connor, issued Dec. 28, 1982, discloses a water recovery process for oil field muds that uses an oil/water separator followed by a filtration/adsorption unit to remove solids and hydrocarbons. The filtration/adsorption unit can contain a filter for further removal of solids and an adsorbant, such as charcoal or granular anthracite, for removal of small amounts of hydrocarbon remaining in the water stream after passing through the oil/water separator. The water then passes to a reverse osmosis unit. U.S. Pat. No. 3,836,458 by Wallis, et al., issued Sept. 17, 1974, discloses a water purification method and apparatus using various filters, including carbon filters, cellulose acetate filters and a reverse osmosis membrane. U.S. Pat. No. 3,498,909 by Littman, issued Mar. 3, 1970, discloses a device for desalination wherein a plurality of porous glass tubes are utilized. Littman also discloses the use of a reverse osmosis membrane upstream of the glass tubes.
When particulate filters are used in purifying a liquid, problems caused by uneven flow through the filters are common. Uneven flow of a contaminated liquid through a particulate filter can result in channels developing in the filter. The channels allow solids and other contaminants normally removed by the filter to substantially bypass the particulate filter material and pass through the unit with the liquid to be recovered. The above-mentioned prior processes do not address or attempt to solve the problem of uneven flow of a contaminated liquid through a particulate filter.
The aforementioned references disclose the use of multiple vessels for holding various types of filters and reverse osmosis membranes as well as the necessary pipes and pumps for moving a contaminated liquid from vessel to vessel. Multiple vessel systems can be cumbersome and expensive due to the number of individual pieces of equipment involved.
Another problem with the devices disclosed in the above cited references is the length of time required for periodic shut downs. Filter devices for purifying contaminated liquids require shutdown in order for filters and/or membranes to be cleaned. Cleaning of filters and membranes is commonly accomplished through back washing or reversing the flow through the system to physically remove contaminants built-up on the filters, as disclosed in Bray, Lerat, and O'Connor. Air scouring has also been used for cleaning particulate filters.
Prior processes for cleaning reverse osmosis membranes include, for example, use of a non-ionic surfactant in combination with a sodium or potassium salt and mineral acid, as disclosed in U.S. Pat. No. 3,827,976, by Stana, et al., issued July 11, 1972. U.S. Pat. No. 3,912,624 by Jennings, issued Oct. 14, 1975, discloses a method for cleaning reverse osmosis membranes whereby pressurized air is applied to the upstream side of the membrane causing impaction of water droplets thereby dislodging deposits on the membrane. U.S. Pat. No. 4,198,293 by Ogawa, et al., issued Apr. 15, 1980, discloses a method for cleaning reverse osmosis membranes in a tubular apparatus by introducing elastic elements to rub off accumulated deposits.
In prior processes the shut down of one purification unit for cleaning effectively shuts down the entire purification process unless duplicate units are used. For example, O'Connor provides for the use of alternate filtration/adsorption units so that one can be backwashed while the other is in operation thereby preventing shut down of the entire process during cleaning.
Therefore, it would be advantageous to provide a purification apparatus which minimizes the problems associated with uneven flow of contaminated liquid through a particulate filter. It would also be advantageous to minimize the complexity and costs associated with a purification apparatus. Furthermore, it would be advantageous if the cleaning of the apparatus could be accomplished with minimum amount of downtime. It would especially be advantageous if all components could be cleaned in a single operation. Additionally, it would be advantageous if the cleaning process were highly efficient in cleaning the various components associated with the apparatus.